The present invention relates to a filtration media and device comprising at least a layer having a structured surface that defines highly ordered fluid pathways.
A variety of filtration devices are used to remove particulate contaminates, including dust particles, mists, smoke particles and the like from gaseous carrier materials, and particularly from air (hereinafter collectively referred to as xe2x80x9cairxe2x80x9d). Certain of these filter devices rely on particle capture based on charges inherently or actively induced on the particles. With the active charge devices or electrofilters generally there is a charge emitter or ionizer that actively transfers charges to the particles. A collection cell or device, that is typically also actively charged or provided with a potential, is coupled with the charging device to capture the charged particles. These electrostatic air filters have demonstrated improved collection efficiencies for small particulate materials as compared to conventional mechanical filtration devices.
Electrofilters are widely used today for industrial gas cleaning in the removal of particles smaller than 20 microns. Electrofilters employ ionization or other charge emitting sources and forces from electric fields to promote the capture of particles in high flow-through, low pressure drop systems. The electrofilters can be either a single-stage device, wherein the ionization source and collection electrode are combined in a single element, or more commonly a two-stage device that employs an upstream ionization source that is independent of a down stream particle collection stage. Functional attributes such as relatively high efficiency and low pressure drop make two-stage electrofilters particularly well suited for in-door air quality enhancement applications. However these devices are relatively expensive, require periodic cleaning (which is often difficult) and can become odorous over time. The collector performance is also negatively impacted by the deposited particles and can deteriorate over time.
In two stage electrofilter devices, particulates are generally charged as the particulate-laden gas stream is passed between a high-voltage electrode and a ground that are maintained at a field strength sufficient to establish a glow discharge or corona between the electrodes. Discharged gas ions and electrons generated in the corona move across the flow stream, colliding with and charging particulate contaminants in the gas stream. This mechanism, which is known as bombardment or field charging, is principally responsible for charging particles greater than 1 micron in size. Particulates smaller than about 0.2 microns are charged by a second mechanism known as diffusion charging, that results from the collection of gas ions on particles through thermal motion of the ions and the Brownian motion of the particles.
If a dielectric or conductive particle is placed in the path of mobile ions a proportion of the surface of each particle will be given a strong electrical charge. That charge is redistributed over the surface of a conductive particle almost instantaneously whereas it is only very slowly redistributed over the surface of a non-conductor particle. Once charged, particulate contaminants are moved toward the collector surface as they enter the particle collection stage. In the absence of mobile ions, conductive particles captured on the collector surface are free to leave the surface because they have shared their charge with the surface. On the other hand, dielectric and/or non-conducting particles that do not readily lose their charge are retained on the collector surface. This attraction force weakens, however, as layers of particles build up and, in effect, create an electrical insulation boundary between particles and the collector surface. These charge decoupling mechanisms, in combination with flow-stream induced dynamic motion at the collector surface, can lead to disassociation of particulate materials from the collector. Once disassociation from the collector surface occurs, the particle is free to reentrain itself in the air stream.
Electrofiltration devices that rely on electrostatic attraction between contaminant particles and charged collector surfaces are generally exemplified by collectors formed from actively charged conductive (metallic or metalized) flat electrode plates separated by dielectric insulators such as described in U.S. Pat. No. 4,234,324 (Dodge, Jr.) or U.S. Pat. No. 4,313,741 (Masuda et. al.). With these devices, inherently charged particles, or particles induced with a charge, such as by an ionizer or charge emitter as described above, are passed between flat charged electrode collector plates. Dodge proposes use of thin metalized Mylar sheets separated by insulating spacers on the ends of the sheets and wound into a roll. These constructions are described as lower cost than conventional metal plates and can be powered by low voltage sources, which, however, require closer spacing of the metalized sheets. This construction allegedly is of a cost that would permit the collector to be discarded rather than requiring periodic cleaning. Additionally, this construction would also eliminate the odor problem. Masuda et. al. also describes the above problems with conventional metal plates and proposes a specific plate design to address the problems of sparking and some of the loss in efficiency problems, but periodic cleaning is still required and odors are still a problem.
In an effort to provide serviceable electrofiltration devices that do not require periodic cleaning, U.S. Pat. No. 3,783,588 (Hudis) describes the use of films of permanently electrically charged polymers that move on rolls into and out of the collector. In this construction, new, uncontaminated, charged film is constantly moved from one roll into the collector space and dirty film is moved out of the collector space onto a collector roll. Periodically the film rolls must be replaced, which would be time consuming, particularly where large numbers of film rolls are employed.
Also used are passively charged disposable filters where the filter media is charged. These provide improved filtration performance relative to particles that have some charge or polarity at relatively low pressure drops. These charged filter media are generally nonwoven or woven fibrous filters where particles impact a face of the media and pass through the fibrous media. Efficiency and lifetime particle capacity are typically increased by increasing the basis weight of the media, which correspondingly increases pressure drop. This pressure drop increase can cause significant problems in situations where a fairly constant flow of air is important, such as some electronic devices, air conditioners and automotive environments.
There has been proposed as a method of decreasing this increase in flow resistance, and associated pressure drop, using filters where the fluid flows over the face of the filter media and not through the media. This is done by creating flow through channel filters where the flow channels sidewalls are formed by otherwise conventional particulate or sorbent filter media. Particles are captured when they contact these filter media sidewalls. As the air flows along the face of the filter media rather than through it, there is generally no dramatic increase in pressure drop over, the filter""s useful life. In view of its increased particle capture capabilities, generally the particulate filtration media used in these constructions are electret charged fibrous media, generally a nonwoven filter media formed of charged fibers. For example, Japanese Kokai 7-144108 (published Jun. 6, 1995) indicates that it is known to form honeycomb shaped filters (e.g., pleated corrugated filter media resembling corrugated cardboard) from electret charged nonwoven filter media. This patent application proposes increasing the long term efficiency of such a filter structure by forming it from a filter media laminate of charged meltblown fiber filter media and charged split fiber filter media (e.g., similar to filter media disclosed in U.S. Pat. No. RE 30,782). Japanese Kokai 7-241491 (published Sep. 19, 1995) proposes a honeycomb filter, as above, where the pleated layers and the flat layers forming the corrugated honeycomb structure are alternating layers of electret charged nonwoven filter media and sorbent filter media (an activated carbon loaded sheet or the like), the activated carbon layer preferably is formed with a liner (e.g., a nonwoven) that may also be electret charged. Japanese Kokai 10-174823 (published Jun. 30, 1998) discloses another honeycomb type filter, as above, where the filter material forming the honeycomb structure is formed from a laminate of an electret charged nonwoven filter layer and an antibacterial filter layer. These honeycomb type filters are described as advantageous for uses where low pressure drop is critical and single pass filtration efficiency is less important; for example, recirculating type filters such as used in air conditioners, room air cleaners or the like. Generally, these honeycomb filters are formed by a process similar to that used to form cardboard where one filter media is pleated and glued at its peaks to a flat layer. The assemblies are then stacked or rolled up where adjacent laminate layers can be joined by glue or hot melt adhesive. The filtration media is charged by conventional techniques prior to forming the honeycomb structures.
A different approach to a flow through type filter is proposed in U.S. Pat. No. 3,550,257 where the charged filtration media is a film rather than a nonwoven filter media. The charged flat films in this patent are separated by spacers strips that are described as open cell foam webs of glass fibers or corrugated Kraft paper. The pressure drop is described as dependent on the porosity of the spacers and the space between the charged dielectric films. Japanese Kokai 56-10314 (published Feb. 2, 1981) discloses a similar structure where a corrugated honeycomb structure is formed with either, or both, the pleated or flat layers are formed from a charged polymeric film (film is defined either as a film or a nonwoven). The layers are adhered by melting the front edges of the multilayer structure together. It is disclosed that the film is imparted with xe2x80x9cwrinklesxe2x80x9d by the folding process. Similar xe2x80x9cfilmxe2x80x9d type honeycomb structures, formed from charged xe2x80x9cfilmsxe2x80x9d, are further disclosed in related Japanese Kokai 56-10312 and 56-10313, both published Feb. 2, 1981.
Improved versions of these flow through channel filters are proposed in PCT publications WO99/65593 and WO00/44472 using film based channel filters where the films have large or high aspect ratio surface structures. These surface structures can either define the channels (WO99/65593) or provide enhanced performance in a channel filter formed by a pleated or corrugated film (WO00/44472).
The present invention provides an improved filtration media or a particle collection element for an electrofiltration apparatus comprising multiple film layers having structured surfaces which structures define particular ordered fluid pathways. The filtration media of the present invention generally comprises a stack of these structured film layers. The structured surfaces defining highly ordered arrays of filter openings and fluid pathways, of a filtration layer, through the assembled filtration media.
The structured surfaces of the film layers may comprise features defining channels that form the fluid pathways, or may comprise features, such as discrete protuberances, that form the fluid pathways with other elements. The filtration media can be produced in a high variety of configurations to meet the filtration requirements of a given application. This variety is manifested in the structured surface feature possibilitiesxe2x80x94discrete channels, open channels, or protuberances; channel configurationsxe2x80x94wide, narrow, xe2x80x98Vxe2x80x99 shaped, and/or sub-channels; stack configurationsxe2x80x94bonded or unbonded, facing layers, non-facing layers, added layers, aligned channels, offset channels, and/or channel patterns; and filter openingsxe2x80x94pore size, pore configuration, or pore pattern. In addition, the layers may be treated for enhanced filtration or other purposes. Generally, the channels formed have a rectilinear cross-section with average channel heights of from 0.1 to 5 mm and average channel aspect ratios of from 0.5 to 10, the aspect ratio being the ratio of the average channel width to height.
The filtration media is formed from at least one polymeric layer having a structured surface defined within or on it. Film layers are configured as a stack with the structured surfaces of the layers defining a plurality of ordered inlets open through a face of the stack and corresponding ordered fluid pathways. The inlets and fluid pathways are formed by the structured surface with a cap layer. The cap layer may be an unstructured layer or a layer with a structured surface.
In a preferred embodiment, the primary flow channels are preferably defined by a series of peaks, each having at least two sidewalls on a film layer. The peaks are separated by a floor, which may have sub-peaks or other sub-structures which can form structures within the primary flow channels. The fluid pathways of a layer within the filtration media is formed at least in part by a structured surface and may be all the same or may be different. Each filtration layer of the filtration media may have the same flow channel configuration, or may be different. The fluid pathways on adjacent filtration layers may be aligned or may be offset.
Additional layers may be added to the stack of film layers. A cap layer may cover a portion of the top of a structured film layer, and additional functional layers may be placed between adjacent layers of the stack. The layers of the stack, may be bonded together. The film layers may be formed from the same or different polymeric materials. The filtration media individual film or other layers may be treated to enhance particle removal or to provide other benefits such as providing oil and water repellency, removing odors, removing organic matter, removing ozone, disinfecting, drying, and introducing fragrance. Treatment generally includes charging of the film layers to form an electret with optional surface coating of certain layers, or the addition of treated layers.
The invention filtration media is particularly useful as a disposable particle collection cell or stage of an electrofiltration apparatus with an ionizer stage. The structured film layer has a first face and a second face, at least one face of the structured film forms, at least in part, flow channels and has high aspect ratio structures over at least a portion of the face forming the flow channels which structures at least in part define the flow channels which in turn define the fluid pathways. A second film layer (comprising the flow channel layer second layer), or a further layer, at least in part, also defines the ordered fluid pathways with the flow channels of the structured film layer. The flow channel layer and the opposing film layers forming the fluid pathways are electret charged.